This invention relates to nucleic acid polymerization and amplification. In particular, it relates to a novel and general method for nucleic acid amplification, in which pyrophosphorolysis and polymerization are serially-coupled. The method has been adapted for allele-specific amplification and can greatly increase the specificity to detect an extremely rare allele in the presence of wild-type alleles. We refer to the method as pyrophosphorolysis activated polymerization (PAP).
The publications and other materials used herein to illuminate the background of the invention or provide additional details respecting the practice, are incorporated by reference, and for convenience are respectively grouped in the appended Bibliography.
Multiple methods for detecting mutations present in less than 10% of cells (i.e. rare alleles) have been developed, including PCR amplification of specific alleles (PASA), peptide nucleic acid (PNA) clamping blocker PCR, allele-specific competitive blocker PCR, mismatch amplification mutation assay (MAMA), restriction fragment-length polymorphism (RFLP)/PCR (Parsons and Heflich, 1997) and QE-PCR (Ronai and Minamoto, 1997). These methods: i) amplify the rare allele selectively, ii) destroy the abundant wild-type allele, or iii) spatially separate the rare allele from the wild-type allele. The specificity -achievable under typical research/clinical conditions is 10−3 (Parsons and Heflich, 1997), although a few publications reported higher specificity of detection (Pourzand and Cerutti, 1993; Knoll et al., 1996). These methods either do not generally achieve the higher specificity or are not suitable for routine analysis.
A robust method of detecting one mutant allele in 104–109 wild-type alleles would be advantageous for many applications including detecting minimal residual disease (recurrence after remission or rare remaining cancer cells in lymph nodes and other neighboring tissues) and measurement of mutation load (the frequency and pattern of somatic mutations present in normal tissues). Individuals with a high mutation load may be at increased risk for cancer due to either environmental exposure or endogenous defects in any of hundreds of genes necessary to maintain the integrity of the genome. For those individuals found to have a high mutation load, clues to etiology can be obtained by defining the mutation pattern.
There are many DNA sequencing methods and their variants, such as the Sanger sequencing using dideoxy termination and denaturing gel electrophoresis (Sanger et al., 1977), Maxam-Gilbert sequencing using chemical cleavage and denaturing gel electrophoresis (Maxam and Gilbert, 1977), pyro-sequencing detecting pyrophosphate (PPi) released during the DNA polymerase reaction (Ronaghi et al., 1998), and sequencing by hybridization (SBH) using oligonucleotides (Lysov et al., 1988; Bains and Smith, 1988; Drmanac et al., 1989; Khrapko et al., 1989; Pevzner et al., 1989; Southern et al., 1992).
There are multiple gel-based methods for scanning for unknown mutations including single stranded conformation polymorphism (SSCP) and the SSCP-hybrid methods of dideoxy fingerprinting (ddF), restriction endonuclease fingerprinting (REF), and Detection Of Virtually All Mutations-SSCP (DOV AM-S), denaturing gradient gel electrophoresis (DGGE), denaturing HPLC (dHPLC) chemical or enzymatic cleavage (Sarkar et al., 1992; Liu and Sommer, 1995; Liu et al., 1999; Myers et al., 1985; Cotton et al., 1988; Liu et al., 1999; Buzin et al., 2000; Spiegelman et al., 2000). DOVAM-S and chemical cleavage reactions have been shown in blinded analyses to identify essentially all mutations (Buzin et al., 2000). dHPLC, which is based on reverse phase chromatography, also may identify essentially all mutations under appropriate conditions (O'Donovan et al., 1998; Oefner and Underhill, 1998; Spiegelman et al., 2000). Efforts are under way to develop general scanning methods with higher throughput.
Sequencing by hybridization (SBH) is being adapted to scanning or resequencing for unknown mutations on microarrays (Southern, 1996). This continues to be a promising area of intense study. However it is not possible as yet to detect most microinsertions and deletions with this approach and the signal to noise ratio for single base changes precludes detection of 5–10% of single nucleotide changes (Hacia, 1999). Alternative approaches warrant exploration.
It is becoming increasingly apparent that in vivo chromatin structure is crucial for mammalian gene regulation and development. Stable changes in chromatin structure often involve changes in methylation and/or changes in histone acetylation. Somatically heritable changes in chromatin structure are commonly called epigenetic changes (Russo and Riggs, 1996) and it is now clear that epigenetic “mistakes” or epimutations are frequently an important contributing factor to the development of cancer (Jones and Laird, 1999).
One of the few methods for assaying in vivo chromatin structure, and the only method with resolution at the single nucleotide level, is ligation-mediated PCR (LM-PCR) (Mueller and Wold, 1989; Pfeifer et al., 1989) and its variant of terminal transferase-mediated PCR (TD-PCR) (Komura and Riggs, 1998). Many aspects of chromatin structure can be determined by LM-PCR, such as the location of methylated cytosine residues, bound transcription factors, or positioned nucleosomes. It is readily apparent that LM-PCR works better with some primer sets than with others. Thus, it is desired to develop a more robust method of measuring chromatin structure.
Thus, it is an object of the present invention to develop alternative methods for amplification of DNA, for sequencing DNA and for analysis of chromatin structure. This object is accomplished by the use of the novel pyrophosphorolysis activated polymerization (PAP) as described herein. PAP has the potential to enhance dramatically the specificity of the amplification of specific alleles, for resequencing DNA and for chromatin structure analysis.